1. Field of the Invention
The present invention relates to an echo cancellor and more particularly to an echo cancellor employing an adaptive digital filter in a communication system.
2. Description of the Related Art
In the conventional echo cancellor using an adaptive digital filter, echo noises are suppressed in a manner described below.
First, an impulse response of an echo path is obtained. On the basis of this result, a pseudo transmission path is established using a transversal filter provided within the adaptive digital filter. This pseudo transmission path is constructed by setting a tap coefficient of the transversal filter. Next, part of a sending signal is inputted in the transversal filter and a pseudo echo mimicking echo noises is acquired from an output from the transversal filter.
The echo noise is suppressed by subtracting this pseudo echo from the sending signal. Since the impulse response of the echo path is changed in some cases, tracing is performed by altering the tap coefficient. The echo noise refers to a noise mixed in the sending signal which is generated when, for example, in a hand-free car phone, a receiving voice emitted from a speaker is reflected off window glass, side walls and the like in a car and, after passing through two or more paths, is received by a microphone. The echo path is a general term of two or more paths for the receiving voice.
However, there are problems as described below to be solved in the conventional echo cancellor. That is, if an ambient noise such as a voice of a user other than those using the hand-free car phone or an alarm of whistle in a vehicle is mixed in the receiving signal, since its transmission path cannot be estimated and tracing using the tap coefficient is impossible, it is difficult to suppress such echo noises accordingly. Moreover, there may be some cases where the tap coefficient diverges, causing the occurrence of a foreign sound.